In recent years, research on the application of plasma treated water to agriculture or medicine has been actively carried out. For further application of plasma-treated water, it is desirable to understand the reaction process and to prepare plasma-treated water of arbitrary rate and concentration. In this study, the plasma treated water prepared by spark discharges in capillary quartz tubes were investigated. The diameter of the capillary tube was changed from 1.0 mm to 3.5 mm, and the change over time of concentrations of NO₂^-, NO₃^-, H₂O₂, and pH were investigated. It was found that the rate of increase in NO₂^- was suppressed as the diameter of the capillary tube decreased. As a result, the treatment time at pH = 4.6, the treatment time when NO₃^- starts to increase, and the treatment time when H₂O₂ starts to decrease, were increased. The reason for the decrease in the rate of increase in NO₂^- was considered that the generation of NO₂ around the plasma was suppressed as the diameter of the capillary tube decreased, and NO₂ was less dissolved in water.